Multiple subduction cycles in the Alpine orogeny, as recorded in single zircon crystals (Rhodope zone, Greece)

Abstract

High- and ultrahigh-pressure metamorphic crustal rocks in orogenic belts provide evidence for subduction into the mantle and subsequent exhumation. The timing of metamorphism(s) of complex high- and ultrahigh-pressure rocks can be registered in the robust mineral zircon, able to preserve different growth generations. Here we present sensitive high resolution ion microprobe (SHRIMP) U–Pb age and REE compositional data from zircon in migmatitic gneisses from the ultrahigh-pressure Kimi unit of the Alpine Rhodope zone (Greece). Single zircon crystals preserve one to two magmatic (one inherited and one syn-magmatic) and two to three metamorphic generations. The 206Pb/238U ages, combined with REE zircon data, mineral inclusions in metamorphic zircon, petrological data from the matrix assemblages, as well as pseudosection calculations are interpreted to reflect Permian crystallization of the protolith of the studied gneisses and subsequent repeated subductions to high pressures over the course of the Alpine orogeny (at ~158Ma, suggested to be close to the time of ultrahigh-pressure metamorphism; at ~74Ma, and possibly also at ~42Ma). This inference stands in contrast to the general thinking about subduction/exhumation cycles during Alpine-type orogenesis along a single path and is in line with recent simulation results about repeated subductions. The recognition of two distinct high-pressure subduction cycles within the Alpine orogeny in a time frame of ~80Ma (and the indication of an additional subduction cycle ~30Ma later) in the same zircon crystal is remarkable. It provides evidence for repeated deep subductions over longer (~80Ma) and possibly also shorter (~30Ma) frequencies and emphasizes the view that lithospheric plate collision/subduction during the Alpine orogeny in the Rhodope zone does not have to be a continuous, protracted process. A regime of repeated subduction/exhumation events involving micro-continental fragments intervening between the European and Adriatic margins may be rather responsible for the different (U)HP age clusters in the Rhodope zone.